As promised, job now done, and almost exactly as above.
I've mounted the unit on the dash top though, running the wires down the edge of the dash, then under the sill plates,
I then removed the rear seat back (7 bolts) and ran through an existing hole in to the boot space.
I did the same method removing both rear lights, but kept the grommet, having drilled the centre out to accept the cables.... I did need to trim the plastic on the cable end to get both through the hole in the body though, rather than enlarging the holes.
The colour seems ok to my eyes, as close as I could hope for really.
Thanks again Yoda, as you say, for 13 quid it would be rude not to.
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